I assume that you are using 20 and 40+ bottles so that they are portable, can be filled at a propane fuel station and transported back home.
In theory the liquid supply concept suggestion makes sense, but portable bottles often don't allow for that concept anymore. Perhaps yours do but mine have an internal valve that prevents this from happening.
A lot of LP / Propane sold in retail contains substantial amounts of butane and other similar fuels. Butane evaporates at right around 30 - 35 F, so what can happen is the propane portion evaporates and leaves the liquid butane in the tank. Essentially in winter conditions, it doesn't evaporate very well. There is some vapor pressure - but not tons.
It is one of the ways that companies that sell "pre-filled" bottles make extra money - only 1/2 is usable, but they charge for the whole "tank".
The other problem is what happens to every propane use - the bottle cools substantially during use. I get this just running a grill.
Since you (probably) cannot access the liquid in the tank, and direct heating with an electric blanket has been frowned upon, I would be tempted to put the tanks into a tank of warm water during use. (40-50 F). Even cold well water is usually this warm, so in theory you could just run cold water in / drain it off to help keep it "warmer".